1. Field of the Invention
The present invention relates to a novel process for the preparation of substituted indenofluorenes, which are used, in particular, in organic, electronic devices.
2. Description of the Prior Art
The use of organic, semiconducting compounds which are capable of emission of light in the visible spectral region in organic electroluminescent devices (OLEDs) is just at the beginning of the market introduction. The general structure of such devices is described, for example, in U.S. Pat. No. 4,539,507, U.S. Pat. No. 5,151,629, EP 0676461 and WO 98/27136.
The organic, semiconducting compounds which are capable of the emission of light in the visible spectral region are, inter alia, also organic compounds and polymers having indenofluorene substructures. In order to improve the applicational properties, it may be advantageous to modify the indenofluorene parent structure, for example by the introduction of suitable substituents.
The prior art discloses processes for the substitution of fluorene parent structures (Kelley et al., J. Chem. Res. Miniprint 1997, 12, 2701-2733; Anemian et al., Chem. Commun. 2002, 15, 1608-1609), in which an alkylation of a fluorene parent structure is carried out in the presence of potassium tert-butoxide in dimethylformamide (DMF). If this process known from the prior art is applied to indenofluorene parent structures, yields of up to 84% and purities of up to 93% are obtained.
The prior art furthermore discloses a two-step process for the substitution of indenofluorene parent structures (Macromolecules 2000, 33, 2016-2020), in which an alkylation of an indenofluorene parent structure is carried out in the presence of n-butyllithium in THF at low temperatures. In this process, purities of up to 99% are achieved; the yield of the two-step process is up to 84%. Owing to the reaction conditions and the reagents selected, the preparation of substituted indenofluorene parent structures is relatively expensive. Furthermore, processes with alkyllithium reagents represent a safety risk on an industrial scale owing to the high combustibility.
There thus continues to be a demand for a process for the preparation of substituted indenofluorene parent structures which on the one hand provides the target compounds in high yield and high purity and on the other hand allows access in an inexpensive and non-critical manner from a safety point of view.